For cheap shtty pistons, rings and gaskets u could look here eBay Australia Shop - BSM Trading Pty Ltd: Quad, Pocket Bike Parts, Dirt Bike
But that 138cc kit looks ok if u wanna spend more on it but id prefer to just update the motor to a 140cc if uv floged the 125 already coz the next problem will be the gearbox which is a bitch to fix unless u know what ur doin.